Groove Formation on Metal Substrates by Nanosecond Laser Removal of Melted Material

Abstract

An effective strategy to produce grooves on carbon steel substrates by nanosecond laser radiation is proposed. The aim is to increase the productivity of grooves creation. In this study, two different modes of laser treatment are compared. The first mode focuses on the evaporation of material, while the second focuses on the formation of melted material and its removal by the action of pressure vapors produced by evaporated material. Within some ranges of processing parameters, the shape of the groove can be linearly controlled. The dependence of the groove depth also has a logarithmic nature when the number of passes is increased. Using the liquid phase mode in some ranges of parameters can reduce the amount of evaporated material in comparison with standard techniques in which the material is removed in the form of gas, and fine dust is emitted.

1. Introduction

Among technological operations involving laser radiation, the ablation of metals does not lose its relevance. Micro-, nano-, pico-, and femtosecond laser sources have been compared in terms of material removal rate when boring holes [1,2]. Nanosecond lasers offer the maximum ablation efficiency per unit of applied energy [1,2,3]. Their pulses can remove metal from the treatment zone in the form of gas and melt drops [4]. The overheating effect of the laser action region leads to the appearance of a melt layer, which can be re-moved by vapor pressure [5]. Therefore, the depth of the crater increases. Studies of the melting pool behavior during laser processing by nanosecond pulsed laser sources may increase the efficiency of the ablation process when the area of material removal is larger than the size of the laser beam.

Different types of relief have been investigated by applying a mathematical model for surface microgeometry formation in laser milling [6]. The general mechanism of periodic relief formation in the framework of a laser track on the surface of titanium and stainless steel was studied, as well as the random or controlled nature of the occurrence. Essentially, during model development, researchers were mindful of the behavior of the liquid phase when a relief appears. It has been shown that control of the liquid phase during the laser casting process can generate grooves and beads with very specific characteristics, which include groove depth, groove width, the horizontal distance between the point of maximum bead height and minimum groove depth, and weld bead height. The movement of the melt material was controlled to obtain a convex relief on the metal surface during laser processing [7,8]. Microcraters and surface diffraction gratings were formed by controlling the behavior of the melt in micro-regions when the metal surface was exposed to pulsed radiation of nanosecond duration with a wavelength of 1.064 µm [7].

Various types of relief with depressions and protrusions of approximately 1 mm in height have been obtained on steel surfaces using a 200 W continuous-wave Yb fiber laser without the use of fillers. Strategy, processing modes and the trajectory of the beam along the surface greatly influenced the formation of the relief. Additionally, a patented process under the trademark Surfi-Sculpt^®, which specifies modes, strategies, and beam paths for obtaining various types of controlled surface topography has been considered [8]. Metal removal is widely used in both industry and research. The creation of relief on a metal surface using a laser source and a scanning system can improve the adhesion properties of the surface [9,10,11].

The creation of grooves on the surface of engineering materials specimen accentuates the absorption characteristics of the surface in the visible and IR ranges or imparts capillary properties to the treated surface [12,13,14]. The creation of the topology using a laser with a scanning system can improve the tribological properties of the steel surface [15]. Studies focused on increasing the efficiency of laser removal of metals when the removal region is much larger than the size of the laser beam spot have not yet been found in the literature.

In this context, this study focuses on the relationship between the input and the investigated parameters at which there is a significant formation and movement of molten material from the processing zone when the metal is irradiated with the focused radiation of a 100 W IR nanosecond laser. The processing speeds required to create a depression in a metal surface were compared to optimize two different laser deepening processes. One strategy ensures preferential evaporation of the material, and the other the formation and removal of molten material from the processing area under the influence of vapor pressure.

2. Materials and Methods

The irradiated substrates consist of structural carbon steel grade St3sp (GOST 3778-77) with a thickness of 1–5 mm. The chemical composition of this steel is shown in

Table 1. Chemical composition of carbon steel grade St3sp.

Element	C	Si	Mn	Ni	S	P	Cr	N	Cu	As
wt%	0.14–0.22	0.05–0.15	0.4–0.65	<0.3	<0.05	<0.04	<0.3	<0.008	<0.3	<0.08

.

The laser processing setup included a YPLN-1-100-100-M nanosecond pulsed fiber laser (IPG Photonics) transmitted with a two-axis galvo head scanner. Then, the laser beam was directed using an F-Theta lens with a scanning field of 100 × 100 mm. The laser spot size was assumed to be 55 microns with Gaussian distribution. The general technical characteristics of the laser system are shown in

Table 2. General technical characteristics of the laser systems.

Source Model	YLPN-1-100-100-M
Architecture	Fiber optic with modulated Q-factor
Wavelength (λ), nm	1064
Max. Output Power, W	100
Pulse Repetition Rate (F), kHz	2–500
Max. Pulse Energy, mJ	1
Pulse Duration (τ), ns	120
Beam Quality, M2	<2
Beam Diameter, µm	55
Max. Scanning Speed, mm/s	10,000

.

The relief parameters were measured and assessed using optical microscopy, and relative measurements were made on the images obtained using a Mars1300-60gc digital camera, as well as the scale of the LOMO MSP-1 microscope. The error in measuring the linear dimensions with the microscope scale was 10 μm. Measurements of the mass of the removed metal were carried out on a UNIGRAM ET-300P-M balance with an error of 0.005 g. The sample mass was measured through successive processing stages. Then, using the method of least squares, the mass of the metal removed during the formation of one depression was calculated. The power of laser radiation exiting the scanning system was measured using an Ophir Photonics power meter. The F-Theta lens was removed, and the laser radiation was fixed in a static spot on the power meter sensor. The radiation was switched on until a stabilized value was shown on the display.

2.1. Choice of Frequency and Power Level

Measurements were made of the power of laser radiation arriving at the sample at different power factors, as well as the pulse repetition rate. Figure 1 shows graphs of the measured power versus frequency at three values of the power factor. The pulse energy was calculated with the following equation:

E_p = P_las/F

(1)

It can be seen in Figure 1 that in the frequency (F) range 0–100 kHz, the laser radiation power (P_las) increases from zero to 100 W or the maximum value, which is determined by the power factor (Pc). Therefore, a frequency in the 100–500 kHz range should be employed to obtain the maximum material removal performance. According to [1], the higher the pulse energy (E_p), the higher the fraction of evaporated metal per 1 mJ. It can be seen in Figure 1b that the energy in the pulse decreases with increasing frequency. Therefore, for the studies, a frequency value of 100 kHz and a power factor of 100% were chosen.

2.2. Separation of Two Processing Modes. Selection of the Region of Velocities

To separate the two processing modes (the evaporation method and the liquid phase method), a simple study was proposed to observe the effect of the degree of pulse overlap on the formation of a liquid phase during metal surface processing. To achieve this, a processing model was proposed and designed in the laser system programming software, represented as a set of parallel segments of the same length. All processing parameters were the same for each segment, except for the beam speed. When moving to the next segment, the speed varied linearly from 10 to 1000 mm/s. As the frequency did not change, the degree of overlapping of the adjacent spots decreased with increasing speed.

Figure 2 displays the video footage of the treatment process of the metal surface with a constant increase in the beam speed. During seconds 2 and 3, the interaction of laser radiation with the metal surface produces sparks, which can be explained by the fact that the interaction of the laser with the surface causes the metal to heat up to a considerable depth.

The vapor pressure is sufficient for splashing out melt drops from the treatment zone. It can be assumed that up to 2 s, the metal heats up, but the channel collapses due to the superiority of capillary forces over the vapor recoil forces and an insignificant ejection of melt drops occurs. Starting from 4 s, the blue glow increases, which indicates the dominant form of steam and low-temperature plasma. Comparing the segments processed on the frames, the characteristics of the regions were obtained. At speeds less than 40 mm/s, the channel collapses; at speeds between 40 and 50 mm/s, melt drops are ejected and at speeds greater than 150 mm/s, mainly evaporation occurs. Thus, the processing mode with the prevalence of melt movement will be considered at speeds of 50–200 mm/s and the evaporation mode at a speed of about 1000 mm/s.

2.3. Research Methodology

The following parameters were used as input parameters: the length of the vectors along which the ray moved (L), the speed of the ray movement (V), the distance between adjacent vectors (R), and the number of processing repetitions (N). Repetitions were performed within the entire set of vectors and not for each vector. The following dimensions of the relief cross-section were used as output parameters: groove depth (D), groove width (P), the horizontal distance between the point of maximum bead height and minimum groove depth (S), and weld bead height (H).

To study the nature of the relief resulting from the liquid phase mode, a technique was proposed with four input and four output parameters. The trajectory along which the laser spot moved on the treated surface was a set of parallel co-directional vectors. Figure 3 presents a diagram of the motion of the spot on the treated surface and of the cross-section of the treated area with an indication of the studied parameters.

2.4. Study of the Formation of Grooves in the Liquid Phase Mode

Previously, the main investigated geometric parameters of the grooves (D, P, S, H) were indicated. For convenience and clarity, dimensionless quantities were introduced. The value D/Lmax (Lmax = 1 mm) characterizes the groove depth as a fraction of the maximum vector length, in this case, equal to 1 mm. The groove shape parameter D/P characterizes the elongation of the groove; with D/P = 1, the groove depth coincides with the width of the inlet hole. The distance of the bead from the groove was described by the value S/Lmax, which characterizes the distance of the groove from the bead as a fraction of the maximum length of the vector, in this case, equal to 1 mm. The bead height was characterized by the ratio of bead height to groove depth H/D.

Figure 4 shows the effect of increasing the distance between vectors (R) at a constant speed (V = 100 mm/s) and constant vector length (L = 1 mm). In the interval of variation of R from 10 to 60 μm, the groove depth decreases approximately from 500 to 200 μm, and the shape of the groove changes from elongated to squared. In the interval of variation of R from 60 to 100 μm, the groove depth practically does not change, and the shape transforms from square to elongated. In other words, an increase in the vector density first increases the width of the groove. Then, after a distance between vectors of 60 µm, it begins to increase both the width and the depth.

When the distance values between the processing lines are between 60 and 100 μm, the action of the vectors can be described by a model in which each vector acts separately, leading to the establishment of a transition point characteristic at R = 60 μm. Similar relief could be obtained if there were a time lag between the vectors, and in the range of variation of R from 60 to 10 μm, the degree of overlap of the vectors begins to play an important role. The mechanism of movement of the molten material has changed. By adding time between vectors, treatment outcomes will differ from treatment without delay. An indirect confirmation of this hypothesis is the fact that the 55 μm beam diameter has a value close to the transition point. The behavior of the dependence of the parameters of the bead also has a “transition point” at R = 60 μm.

The distance between the bead and the groove in the range of R change from 60 to 100 μm remains unchanged and equal to approximately half the length of the vector along which the processing takes place. After passing the boundary R = 60 µm, the distance S grows from about 300 to 500 µm. The ratio of the height of the bead to the depth of the groove first varies in the interval 1–1.5, and after passing the boundary R = 60 μm it sharply decreases to values of 0.4–0.6.

It should be noted that when the distance between the vectors is more than 100 microns, there are separate indentations from each vector. Figure 5 shows from above how the transition from individual grooves to a single groove occurs. At a distance between grooves of 125 µm, thin bridges remain between the depressions, and at a distance of 100 µm, the bridges are no longer observed.

Figure 6 shows the effect of the vector length (L) on the parameters understudy (D, P, S, H) at a speed V = 100 mm/s and a constant distance between vectors R = 50 μm. It can be seen in Figure 6c that in the interval when L changes from 0.1 to 0.6 mm, the groove depth increases and the groove lengthens. In the interval from 0.6 mm to 1 mm, the depth decreases slightly, and the width of the inlet hole increases. Starting from a vector length of 0.3 mm, the ratio of the height of the bead to the depth of the groove and the distance between the groove and the bead increases almost monotonically.

Figure 7 shows the effect of the beam speed (V) on the parameters under study (D, P, S, H) for a constant vector length (L = 1 mm) and a constant distance between the vectors (R = 50 μm). It can be seen in Figure 7a that at speeds from 50 to 200 mm/s, the groove depth decreases almost monotonically and an elongated shape along the vertical axis transitions to an elongated shape along the horizontal axis.

A “square” section of the groove was observed at speed of 125 mm/s. The distance between the groove and the bead practically did not depend on the speed and fluctuated around 0.8 mm, and the ratio of the bead height to the groove depth increased almost monotonically with an increase in the speed from 50 to 200 mm/s.

It should be noted that the cross-sections of the grooves did not consider that, at a speed above 200 mm/s, the grooves were unstable and, in some places, contained frozen melt droplets. To demonstrate this effect, Figure 8 shows a top view photograph of the grooves obtained at speeds above 200 mm/s.

Figure 9 shows the effect of the number of passes (N) on the parameters under study (D, P, S, H) for a constant vector length (L = 1 mm) and three distances between the vectors (R = 10, 50, 100 μm).

Approximating curves of the form y (x) = Log (x) + y0 have been added to the graphs of dependences. Additionally, for clarity, the graphs were built on a semi-logarithmic scale. It can be seen from the figures that the dependence of the groove depth on the number of passes is fairly well-described by the logarithmic dependence at different distances between the vectors.

2.5. Obtaining an Annular Groove by Two Methods

Using the liquid phase mode, an annular-triangular cross-section deepening was created. In a graphical editor, a control program for the path of the rays was created, presented in Figure 10a. The ray path consisted of four sets of vectors 1 and 0.8 mm long, arranged radially. All vectors are directed to a construction circle with a diameter of 2.25 mm. Moreover, the ends of the vectors lie on this circle. This is shown in Figure 10 by the direction of the four vectors.

The path of the rays ensures the movement of the liquid phase to both sides of this circle. Therefore, two annular beads of solidified metal are formed on the surface of the sample near the groove, which can be seen in Figure 10b,c. The choice of the distance between the vectors, the length of the vectors, and the number of passes were determined based on the results of the previous analysis in this section.

Figure 11 shows how the appearance of the cross-section of the annular groove and the groove depth changes when the speed of vectors changes, and the graph of the dependence of the maximum groove depth of the annular groove on the speed of the beam along the surface of the sample. In the range of speeds from 90 to 140 mm/s, an almost linear dependence of the groove depth on the speed can be observed from the figure.

The required geometric parameters of the annular groove were obtained with the following processing parameters: V = 90 mm/s and N = 2; the length of the vectors of the first pass is 1 mm; while that of the second pass is 0.8 mm.

For comparison of processing conditions, an annular recess was also formed using the evaporation method. The processing parameters were as follows: power factor 100%, scanning speed 3000 mm/s, pulse repetition rate 100 kHz and scanning density 100 lines per mm.

Figure 12 compares the appearance of the grooves obtained by the two methods. It can be seen from the figure that the annular grooves were of the same depth, but in the case of the evaporation mode, there is practically no remelted metal at the edge of the groove and the walls are smoother.

2.6. Measuring the Mass of Material Removed by Two Methods

The mass of the material removed during the formation of the annular depression in the two above-described modes was measured. Figure 13 shows a photograph of a metal plate with applied annular depressions, as well as graphs of the dependences of the mass of the removed material on the number of formed depressions on the surface of the sample.

Graphs indicate the masses of the removed metal during the formation of one annular depression. This value was calculated by the least-squares method. For the liquid phase mode, twice as many measurements were required, as the mass of the removed metal turned out to be an order of magnitude less than in the case of the evaporation mode, which is clearly seen from the graphs.

3. Results and Discussion

3.1. Recess Shape

The advantage of the evaporation method is the fact that the depressions can be of various shapes in diameter. It is only necessary to fulfill the condition for the access of the laser beam to the treatment site without crossing the existing wall. By evaporating the metal layer by layer, one can even form a bas-relief [16]. However, the shape of the resulting depressions is influenced by the edge effect. The fact is that under normal scanning conditions, when using nanosecond radiation, the angle at the walls relative to the surface is 5–10 degrees.

This effect is explained by the fact that at the edge of the depression the area of the region irradiated by the laser increases, and the power density correspondingly decreases, which leads to the evaporation of a smaller amount of metal. The effect is summed up for all layers, as a result, the wall turns out to be inclined instead of a sheer one. Moreover, the inclined wall increases the proportion of reflected radiation.

When using picosecond emitters, the angle becomes even larger: 25–50 degrees [17]. With the use of special scanning systems, cylindrical holes can be obtained with sheer walls with a high quality of the wall surface [18].

At this stage of research, the liquid phase method allows only V-shaped depressions to form in the section (Figure 4, Figure 6, Figure 7 and Figure 9), and because of the melt adhering to the treated surface, obtaining a bas-relief on the metal surface is not possible.

Nevertheless, in some cases, cross-sections close to square (Figure 4) or rectangular (Figure 6) were obtained, but so far, it has not been possible to regulate the geometric parameters of recesses with such cross-sections.

When using the number of passes as a control parameter, a logarithmic dependence of the groove depth on the number of passes was observed (Figure 9). When the annular depression was formed, an almost linear dependence of the groove depth on the speed of the beam was observed (Figure 10).

3.2. Time of Processing

The actual performance of the evaporation method depends on not only the power of the laser source, but also the capabilities of the scanning system. The time to obtain an annular depression depends both on the actual speed of the beam and on the cost of acceleration and deceleration of the servos on which the mirrors of the scanning system are mounted.

The time spent on the formation of the annular depression using the evaporation mode was 43 s. However, the processing time of a cylindrical depression of the same volume was just 15 s. The parasitic time spent on acceleration and deceleration of the scanning system is directly proportional to the perimeter of the processed figure, and the useful time spent on directly processing is directly proportional to the area of the figure. In the case of a cylinder, the ratio of the perimeter to the cross-sectional area of the cylinder (circle) is minimal. That is, 15 s is the time spent, in principle, on the formation of a ring in an evaporative mode. A more accurate value of the useful time can be obtained by approximating the dependence of the processing time of cylinders of the same volume on their diameter.

The formation time of the annular depression in the case of treatment in the liquid phase was found to be 5 s. The time to obtain a depression in the case of the liquid phase mode depends only slightly on the scanning system; therefore, the efficiency of the entire system, including both the laser source and the scanners, grows purely instrumental. If we compare the net processing time, it is noticeable that even if the scanning system is close to ideal, the evaporation mode is three times slower than the liquid phase mode. The gain can be explained by the fact that in the case of the evaporation regime, the laser radiation energy is spent mainly on the evaporation of the metal, while in the liquid phase regime, it is spent on heating to the melting point and moving the melt upward from the treated area. This is confirmed by experiments on measuring the removed mass of metal during the formation of depressions (Figure 13).

3.3. Scanning Head Position

The position of the scanning head, in which the treated surface is inside the focal plane of the F-theta lens, plays an essential role in the process of evaporation.

The bottom of the groove deepens, and it becomes necessary to move the scanning head down manually or to use an electric drive (to move the sample up). It is widely known that if the head is not moved, a situation may arise when, due to the position of the treated surface moving out of focus, the power density in the spot of laser radiation is insufficient to evaporate the same amount of metal as in the previous stages of processing.

Hence, the radiation energy is spent not on evaporation, but on heating the plate. Consequently, the practical performance turns out to be lower than the theoretical one. For the liquid phase method, the position of the scanning head affects the final result to a lesser extent. The extent of this influence needs to be established in future studies. In current studies, the scanning head did not move when using the liquid mode.

3.4. The Surface Quality of the Walls of the Recesses

Obviously, the surface quality of the walls of the depression in the case of the evaporation regime is determined by the spot size and pulse energy, while in the case of the liquid phase regime—by the dynamics of the liquid phase and gas, as well as by the surface tension of the melt.

There are also studies of the influence of the strategy of filling with scanning lines during multi-pass evaporation [19], which indicate that the roughness can decrease by 2.5–3 times when changing the step of the angle of rotation of parallel scanning lines. Improving the surface quality for the evaporation regime has been a well-known problem for a long time.

Currently, the modern level of technology for obtaining a bas-relief is determined by the quality of the surface polishing for various metals and alloys [20,21,22].

The removal mechanism of metal from the surface in the liquid phase resembles gas laser cutting. In this case, evaporating metal vapors act as the blowing gas. Laser and gas-laser cutting of metals developed rapidly almost immediately with the advent of lasers; therefore, many theoretical and experimental studies have been carried out in this area.

This approach holds hope for an improvement in the method of metal removal from the surface of the plate in the liquid phase concerning to the quality of the wall of the machined depression. At this stage, the walls of the depressions obtained in the liquid phase mode are wavy and uneven in the cross-section (Figure 12).

3.5. Environmental Friendliness

Metal processing with the liquid phase method has a positive environmental impact, since the mass of the extracted metal is an order of magnitude less than in the case of the evaporation method, as most of the extracted metal is in the form of droplets instead of gas. The droplets are comparable in size to those found in laser cutting or turning a metal tool on a grinding machine.

The processing parameters when using the liquid phase mode in some ranges of parameters linearly affect the main geometric parameters of the problem; therefore, the use of the method is simplified to obtain depressions with a V-shaped section.

4. Conclusions

Scanning speed plays a key role in the formation of molten material during laser processing. The influence of input parameters (R, L, V, N) on the cross-sectional shape of the relief was investigated. In some ranges of processing parameters, it is possible to linearly control the groove shape. The logarithmic nature of the dependence of the groove depth was observed with a modification in the number of passes.

The process of transferring molten metal has a random nature and, in some cases, leads to a significant deviation of the experimental data, which could be mistaken for repeating effect.

Based on the studied patterns, a new strategy for obtaining depressions on the metal surface was formulated. A comparison was made between the performance and the environmental impact of this strategy and that of the widely used standard strategy, on the evaporation mechanism using a practical method.

The new processing strategy allows increasing the productivity of grooves creation by 3–10 times. When using the new strategy, less metal is removed by an order of magnitude. Moreover, the metal is removed in the form of large droplets of the melt, in comparison with the standard technique, in which the gas evaporates and fine dust is emitted, which has a positive effect on the environment.